Porogens for porous silica dielectric for integral circuit applications

ABSTRACT

The invention relates to the production of nanoporous silica dielectric films and to semiconductor devices and integrated circuits comprising these improved films. The nanoporous films of the invention are prepared using silicon containing pre-polymers and are prepared by a process that allows crosslinking at lowered gel temperatures by means of a metal-ion-free onium or nucleophile catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of nanoporous silicadielectric films and to semiconductor devices and integrated circuitscomprising these improved films. The nanoporous films of the inventionare prepared using silicon containing pre-polymers and are prepared bythe use of double end-capped porogens to prevent chemical attachment ofthe porogen to the Si-network. As a result essentially all the availablesilanol (Si—OH) groups can be cross-linked to give a rigid networkbefore the removal of the porogen, thus producing a nanoporous film withfew silanol groups.

2. Description of the Related Art

As feature sizes in integrated circuits are reduced to below 0.15 μm andbelow, in new generations of electronic devices, problems withinterconnect RC delay, power consumption and signal cross-talk havebecome increasingly difficult to resolve. One of the solutions toovercome these difficulties is to develop materials of dielectricconstant less than about 2.5 for interlevel dielectric (ILD) andintermetal dielectric (IMD) applications. While there have been previousefforts to apply low dielectric constant materials to integratedcircuits, there remains a longstanding need in the art for furtherimprovements in processing methods and in the optimization of both thedielectric and mechanical properties of such materials used in themanufacture of integrated circuits.

One type of material with a low dielectric constant is nanoporous silicafilms prepared from silicon containing pre-polymers by a spin-on sol-geltechnique. Air has a dielectric constant of 1, and when air isintroduced into a suitable silica material having a nanometer-scale porestructure, such films can be prepared with relatively low dielectricconstants (“k”). Nanoporous silica materials are attractive becausesimilar precursors, including organic-substituted silanes, such astetraacetoxysilane (TAS)/methyltriacetoxysilane (MTAS)-derived siliconpolymers are used as the base matrix and are used for the currentlyemployed spin-on-glasses (“S.O.G.”) and chemical vapor deposition(“CVD”) of silica SiO₂. Such materials have demonstrated high mechanicalstrength as indicated by modulus and stud pull data. Mechanicalproperties can be optimized by controlling the pore size distribution ofthe porous film. Nanoporous silica materials are attractive because itis possible to control the pore size, and hence the density, mechanicalstrength and dielectric constant of the resulting film material. Inaddition to a low k, nanoporous films offer other advantages includingthermal stability to 900° C.; substantially small pore size, i.e., atleast an order of magnitude smaller in scale than the microelectronicfeatures of the integrated circuit; preparation from materials such assilica and tetraethoxysilane (TEOS) that are widely used insemiconductors; the ability to “tune” the dielectric constant ofnanoporous silica over a wide range; and deposition of a nanoporous filmcan be achieved using tools similar to those employed for conventionalS.O.G. processing.

High porosity in silica materials leads to a lower dielectric constantthan would otherwise be available from the same materials in nonporousform. An additional advantage, is that additional compositions andprocesses may be employed to produce nanoporous films while varying therelative density of the material. Other materials requirements includethe need to have all pores substantially smaller than circuit featuresizes, the need to manage the strength decrease associated withporosity, and the role of surface chemistry on dielectric constant andenvironmental stability.

Density (or the inverse, porosity) is the key parameter of nanoporousfilms that controls the dielectric constant of the material, and thisproperty is readily varied over a continuous spectrum from the extremesof an air gap at a porosity of 100% to a dense silica with a porosity of0%. As density increases, dielectric constant and mechanical strengthincrease but the degree of porosity decreases, and vice versa. Thissuggests that the density range of nanoporous films must be optimallybalanced between the desired range of low dielectric constant and themechanical properties acceptable for the desired application.

Nanoporous silica films have previously been fabricated by a number ofmethods. For example, nanoporous films have been prepared using amixture of a solvent and a silica precursor, which is deposited on asubstrate suitable for the purpose. Usually, a precursor in the form of,e.g., a spin-on-glass composition is applied to a substrate, and thenpolymerized in such a way as to form a dielectric film comprisingnanometer-scale voids.

When forming such nanoporous films, e.g., by spin-coating, the filmcoating is typically catalyzed with an acid or base catalyst and waterto cause polymerization/gelation (“aging”) during an initial heatingstep. In order to achieve maximum strength through pore size selection,a low molecular weight porogen is used.

U.S. Pat. No. 5,895,263 describes forming a nanoporous silica dielectricfilm on a substrate, e.g., a wafer, by applying a composition comprisingdecomposable polymer and organic polysilica i.e., including condensed orpolymerized silicon polymer, heating the composition to further condensethe polysilica, and decomposing the decomposable polymer to form aporous dielectric layer. This process, like many of the previouslyemployed methods of forming nanoporous films on semiconductors, has thedisadvantage of requiring heating for both the aging or condensingprocess, and for the removal of a polymer to form the nanoporous film.Furthermore, there is a disadvantage that organic polysilica, containedin a precursor solution, tends to increase in molecular weight after thesolution is prepared; consequently, the viscosity of such precursorsolutions increases during storage, and the thickness of films made fromstored solutions will increase as the age of the solution increases. Theinstability of organic polysilica thus requires short shelf life, coldstorage, and fine tuning of the coating parameters to achieve consistentfilm properties in a microelectronics/integrated circuit manufacturingprocess.

Formation of a stable porous structure relies on the condition that theporogen removal temperature is higher than the crosslinking temperature(or the gel temperature) of the matrix material. It was found that astable nanoporous structure of less than 10 nm average pore sizediameter cannot be produced when the concentration of the alkali cationsuch as sodium is below 200-300 parts per billion (ppb) level in thespin-on solution. However, stringent requirement for low metalconcentration must be met for IC applications. The general practice isto have metal concentration below 50 ppb in the spin-on solution.Therefore, there is a need to develop a low metal nanoporous silica filmthat can consistently give dielectric constant less than 2.5 and averagepore size diameter less than about 10 nm in diameter.

In the past, in order to achieve maximum strength through pore sizeselection, low molecular weight polyethylene glycol monomethyl ether waschosen as the porogen. Formation of a stable porous structure relies onthe condition that the porogen removal temperature is higher than thecross-linking temperature (or the gel temperature) of the matrixmaterial. It was observed that such porogen could chemically react withthe Si-network, capping the free silanol groups that are involved incrossing linking reactions during the process. Such species decompose togive un-wanted isolated Si—OH groups after the final curing stage thatis taken place at a much higher temperature. The changes inhydrophilicity resulting from silanol groups detrimentally impact thedielectric properties. Thus, in order to obtain dielectric materialswith low and stable k values, it is desirable to minimize the amount ofisolated silanol groups present in the final film. Further more, havingfree silanol groups also leads to un-desirable out-gassing in the ICintegration. The stringent requirement for low out-gassing and stable kmust be met for IC applications. The general practice is to obtainhydrophobic films. Therefore, there is a need to develop a hydrophobicnanoporous silica film that can consistently give dielectric constantless than 2.2 and absorb as little moisture as possible. Furthermore, itwas widely assumed that pores are formed as the result of chemicalattachment of porogens onto the silica network.

It has now been found that chemical attachment is not necessary to formporous silica. It has now been found that through the use of a doubleend-capped polyethylene oxide porogen, the formation of porous silicaresulting from a physical blending of porogen and silicon pre-polymergives a more hydrophobic film as suggested by its lower delta k valuebetween films at ambient and after heating. The effect of doubleend-capped porogens, such as poly(ethylene glycol)dimethyl ether, is toprevent any chemical attachment of the porogen to the Si-network so thatno additional silanol will be generated during the removal of theporogen and the existing silanol groups will be cross-linked to fullextent possible, thus producing a nanoporous film with few, if any,silanol groups. Through the additional use of onium ions or nucleophilesthe formation of a porous silica network at lower temperature in a lowmetal spin-on formulation can be facilitated. The effect of the oniumions or nucleophiles is to lower the gel temperature so that the rigidnetwork is set in before the removal of the porogen, thus producing ananoporous film without the presence of an alkali ion. The function ofthe porogen is to control the pore size and to readily decompose afterthe formation of stable pores. Other side-reactions that prevent theextent of the cross-linking of sol-gel reactions are minimized.

SUMMARY OF THE INVENTION

The invention provides a method of producing a nanoporous silicadielectric film comprising

-   (a) preparing a composition comprising a silicon containing    pre-polymer, a metal-ion-free catalyst selected from the group    consisting of onium compounds and nucleophiles; and a porogen which    does not bond to the silicon containing pre-polymer;-   (b) coating a substrate with the composition to form a film,-   (c) crosslinking the composition to produce a gelled film, and-   (d) heating the gelled film at a temperature and for a duration    effective to remove substantially all of said porogen.

The invention also provides a nanoporous dielectric film produced on asubstrate by the above method, as well as a semiconductor device, suchas an integrated circuit comprising the nanoporous dielectric film.

The invention also provides porogen which do not bond to a siliconcontaining pre-polymer selected from the group consisting ofpoly(alkylene)diether, poly(arylene)diether, poly(cyclic glycol)diether,Crown ethers, fully end-capped polyalkylene oxides, fully end-cappedpolyarylene oxides, polynorbene, and combinations thereof.

The invention further provides a composition comprising a siliconcontaining pre-polymer, and a porogen which does not bond to the siliconcontaining pre-polymer and is selected from the group consisting ofpoly(alkylene)diether, poly(arylene)diether, poly(cyclic glycol)diether,Crown ethers, fully end-capped polyalkylene oxides, fully end-cappedpolyarylene oxides, polynorbene, and combinations thereof.

The invention still further provides a method of controlling the poresize of a porous silica film, comprising

-   (a) preparing a composition comprising a silicon containing    pre-polymer, a metal-ion-free catalyst selected from the group    consisting of onium compounds and nucleophiles; and a porogen;-   (b) coating a substrate with the composition to form a film,-   (c) crosslinking the composition to produce a gelled film, and-   (d) heating the gelled film at a temperature and for a duration    effective to remove substantially all of said porogen;    the method comprising using a porogen which does not bond to the    silicon containing pre-polymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Accordingly, nanoporous silica dielectric films having a dielectricconstant, or k value, ranging from about 3 or below, preferably about2.5 or below, can be produced by the methods of the invention.Typically, silicon-based dielectric films, including nanoporous silicadielectric films, are prepared from a composition comprising a suitablesilicon containing pre-polymer, blended with a porogen which does notbond to the silicon containing pre-polymer and a metal-ion-free catalystwhich may be an onium compound or a nucleophile. One or more optionalsolvents, other porogens and/or other components may also be included.The dielectric precursor composition is applied to a substrate suitable,e.g., for production of a semiconductor device, such as an integratedcircuit (“IC”), by any art-known method to form a film. The compositionis then crosslinked, such as by heating to produce a gelled film. Thegelled film is then heated at a higher temperature to removesubstantially all of the porogen.

The films produced by the processes of the invention have a number ofadvantages over those previously known to the art, including producing ananoporous film with few, if any, silanol groups, more silicon methylgroups, improved mechanical strength, that enables the produced film towithstand the further processing steps required to prepare asemiconductor device on the treated substrate, and a low and stabledielectric constant. The property of a stable dielectric constant isadvantageously achieved without the need for further surfacemodification steps to render the film surface hydrophobic, as wasformerly required by a number of processes for forming nanoporous silicadielectric films. Instead, the silica dielectric films as produced bythe processes of the invention are sufficiently hydrophobic as initiallyformed.

The processes of the invention provided for a nanometer scale diameterpore size, which is also uniform in size distribution. The resultingnanoporous silica film typically has a dielectric constant of about 3 orbelow, more typically in the range of from about 1.3 to about 3.0, andmost typically from about 1.7 to about 2.5. The film typically has anaverage pore diameter ranging from about 1 nm to about 30 nm, or morepreferably from about 1 nm to about 10 nm and typically from about 1 runto about 5 nm. The film typically has a void volume of from about 5% toabout 80% based on the total volume of the film.

It should be understood that the term nanoporous dielectric films, isintended to refer to dielectric films prepared by the inventive methodsfrom an organic or inorganic glass base material, e.g., any suitablesilicon-based material. Additionally, the term “aging” refers togelling, condensing, or polymerization, of the combined silica-basedprecursor composition on the substrate after deposition. The term“curing” refers to the removal of residual silanol (Si—OH) groups,removal of residual water, and the process of making the film morestable during subsequent processes of the microelectronic manufacturingprocess. The curing process is performed after gelling, typically by theapplication of heat, although any other art-known form of curing may beemployed, e.g., by the application of energy in the form of an electronbeam, ultraviolet radiation, and the like.

Dielectric films, e.g., interlevel dielectric coatings, are preparedfrom suitable compositions applied to a substrate. Art-known methods forapplying the dielectric precursor composition, include, but are notlimited to, spin-coating, dip coating, brushing, rolling, sprayingand/or by chemical vapor deposition. Prior to application of the basematerials to form the dielectric film, the substrate surface isoptionally prepared for coating by standard, art-known cleaning methods.The coating is then processed to achieve the desired type andconsistency of dielectric coating, wherein the processing steps areselected to be appropriate for the selected precursor and the desiredfinal product. Further details of the inventive methods and compositionsare provided below.

A “substrate” as used herein includes any suitable composition formedbefore a nanoporous silica film of the invention is applied to and/orformed on that composition. For example, a substrate is typically asilicon wafer suitable for producing an integrated circuit, and the basematerial from which the nanoporous silica film is formed is applied ontothe substrate by conventional methods, including, but not limited to,the art-known methods of spin-coating, dip coating, brushing, rolling,and/or spraying. Prior to application of the base materials to form thenanoporous silica film, the substrate surface is optionally prepared forcoating by standard, art-known cleaning methods.

Substrates contemplated herein may comprise any desirable substantiallysolid material. Particularly desirable substrate layers comprise films,glass, ceramic, plastic, metal or coated metal, or composite material.In preferred embodiments, the substrate comprises a silicon or galliumarsenide die or wafer surface, a packaging surface such as found in acopper, silver, nickel or gold plated leadframe, a copper surface suchas found in a circuit board or package interconnect trace, a via-wall orstiffener interface (“copper” includes considerations of bare copper andits oxides), a polymer-based packaging or board interface such as foundin a polyimide-based flex package, lead or other metal alloy solder ballsurface, glass and polymers. Useful substrates include silicon, siliconnitride, silicon oxide, silicon oxycarbide, silicon dioxide, siliconcarbide, silicon oxynitride, titanium nitride, tantalum nitride,tungsten nitride, aluminum, copper, tantalum, organosiloxanes, organosilicon glass, and fluorinated silicon glass. In other embodiments, thesubstrate comprises a material common in the packaging and circuit boardindustries such as silicon, copper, glass, and polymers. The circuitboard made up of the present composition will have mounted on itssurface patterns for various electrical conductor circuits. The circuitboard may include various reinforcements, such as woven non-conductingfibers or glass cloth. Such circuit boards may be single sided, as wellas double sided. Suitable substrates for the present inventionnon-exclusively include semiconductor materials such as gallium arsenide(“GaAs”), silicon and compositions containing silicon such ascrystalline silicon, polysilicon, amorphous silicon, epitaxial silicon,and silicon dioxide (“SiO₂”) and mixtures thereof.

On the surface of the substrate is an optional pattern of raised lines,such as metal, oxide, nitride or oxynitride lines which are formed bywell known lithographic techniques. Suitable materials for the linesinclude silica, silicon nitride, titanium nitride, tantalum nitride,aluminum, aluminum alloys, copper, copper alloys, tantalum, tungsten andsilicon oxynitride. Useful metallic targets for making these lines aretaught in commonly assigned U.S. Pat. Nos. 5,780,755; 6,238,494;6,331,233B1; and 6,348,139B1 and are commercially available fromHoneywell International Inc. These lines form the conductors orinsulators of an integrated circuit. Such are typically closelyseparated from one another at distances of about 20 micrometers or less,preferably 1 micrometer or less, and more preferably from about 0.05 toabout 1 micrometer. Other optional features of the surface of a suitablesubstrate include an oxide layer, such as an oxide layer formed byheating a silicon wafer in air, or more preferably, an SiO₂ oxide layerformed by chemical vapor deposition of such art-recognized materials as,e.g., plasma enhanced tetraethoxysilane oxide (“PETEOS”), plasmaenhanced silane oxide (“PE silane”) and combinations thereof, as well asone or more previously formed nanoporous silica dielectric films.

The nanoporous silica film of the invention can be applied so as tocover and/or lie between such optional electronic surface features,e.g., circuit elements and/or conduction pathways that may have beenpreviously formed features of the substrate. Such optional substratefeatures can also be applied above the nanoporous silica film of theinvention in at least one additional layer, so that the low dielectricfilm serves to insulate one or more, or a plurality of electricallyand/or electronically functional layers of the resulting integratedcircuit. Thus, a substrate according to the invention optionallyincludes a silicon material that is formed over or adjacent to ananoporous silica film of the invention, during the manufacture of amultilayer and/or multicomponent integrated circuit. In a furtheroption, a substrate bearing a nanoporous silica film or films accordingto the invention can be further covered with any art known non-porousinsulation layer, e.g., a glass cap layer.

The crosslinkable composition employed for forming nanoporous silicadielectric films according to the invention includes one or more siliconcontaining prepolymers that are readily condensed. It should have atleast two reactive groups that can be hydrolyzed. Such reactive groupsinclude, alkoxy (RO), acetoxy (AcO), etc. Without being bound by anytheory or hypothesis as to how the methods and compositions of theinvention are achieved, it is believed that water hydrolyzes thereactive groups on the silicon monomers to form Si—OH groups (silanols).The latter will undergo condensation reactions with other silanols orwith other reactive groups, as illustrated by the following formulas:Si—OH+HO—Si→Si—O—Si+H₂OSi—OH+RO—Si→Si—O—Si+ROHSi—OH+AcO—Si→Si—O—Si+AcOHSi—OAc+AcO—Si→Si—O—Si+Ac₂OR=alkyl or arylAc=acyl(CH₃CO)

These condensation reactions lead to formation of silicon containingpolymers. In one embodiment of the invention, the prepolymer includes acompound, or any combination of compounds, denoted by Formula I:Rx-Si-Ly  (Formula I)wherein x is an integer ranging from 0 to about 2 and y is 4-x, aninteger ranging from about 2 to about 4,R is independently alkyl, aryl, hydrogen, alkylene, arylene and/orcombinations of these,L is independently selected and is an electronegative group, e.g.,alkoxy, carboxyl, halide, isocyanato and/or combinations of these.

Particularly useful prepolymers are those provided by Formula I when xranges from about 0 to about 2, y ranges from about 2 to about 4, R isalkyl or aryl or H, and L is an electronegative group, and wherein therate of hydrolysis of the Si-L bond is greater than the rate ofhydrolysis of the Si—OCH₂CH₃ bond. Thus, for the following reactionsdesignated as (a) and (b):(a) Si-L+H₂O→Si—OH+HL(b) Si—OCH₂CH₃+H₂O →Si—OH+HOCH₂CH₃

The rate of (a) is greater than rate of (b).

Examples of suitable compounds according to Formula I include, but arenot limited to:

-   -   Si(OCH₂CF₃)₄ tetrakis(2,2,2-trifluoroethoxy)silane,    -   Si(OCOCF₃)₄ tetrakis(trifluoroacetoxy)silane*,    -   Si(OCN)₄ tetraisocyanatosilane,    -   CH₃Si(OCH₂CF₃)₃ tris(2,2,2-trifluoroethoxy)methylsilane,    -   CH₃Si(OCOCF₃)₃ tris(trifluoroacetoxy)methylsilane*,    -   CH₃Si(OCN)₃ methyltriisocyanatosilane, and or combinations of        any of the above. [* These generate acid catalyst upon exposure        of water]

In another embodiment of the invention, the composition includes apolymer synthesized from compounds denoted by Formula I by way ofhydrolysis and condensation reactions, wherein the number averagemolecular weight ranges from about 150 to about 300,000 amu, or moretypically from about 150 to about 10,000 amu.

In a further embodiment of the invention, silicon-containing prepolymersuseful according to the invention include organosilanes, including, forexample, alkoxysilanes according to Formula II:

Optionally, Formula II is an alkoxysilane wherein at least 2 of the Rgroups are independently C₁ to C₄ alkoxy groups, and the balance, ifany, are independently selected from the group consisting of hydrogen,alkyl, phenyl, halogen, substituted phenyl. For purposes of thisinvention, the term alkoxy includes any other organic groups which canbe readily cleaved from silicon at temperatures near room temperature byhydrolysis. R groups can be ethylene glycoxy or propylene glycoxy or thelike, but preferably all four R groups are methoxy, ethoxy, propoxy orbutoxy. The most preferred alkoxysilanes nonexclusively includetetraethoxysilane (TEOS) and tetramethoxysilane.

In a further option, for instance, the prepolymer can also be analkylalkoxysilane as described by Formula I, but instead, at least 2 ofthe R groups are independently C₁ to C₄ alkylalkoxy groups wherein thealkyl moiety is C₁ to C₄ alkyl and the alkoxy moiety is C₁ to C₆ alkoxy,or ether-alkoxy groups; and the balance, if any, are independentlyselected from the group consisting of hydrogen, alkyl, phenyl, halogen,substituted phenyl. In one preferred embodiment each R is methoxy,ethoxy or propoxy. In another preferred embodiment at least two R groupsare alkylalkoxy groups wherein the alkyl moiety is C₁ to C₄ alkyl andthe alkoxy moiety is C₁ to C₆ alkoxy. In yet another preferredembodiment for a vapor phase precursor, at least two R groups areether-alkoxy groups of the formula (C₁ to C₆ alkoxy)_(n) wherein n is 2to 6.

Preferred silicon containing prepolymers include, for example, any or acombination of alkoxysilanes such as tetraethoxysilane,tetrapropoxysilane, tetraisopropoxysilane, tetra(methoxyethoxy)silane,tetra(methoxyethoxyethoxy)silane which have four groups which may behydrolyzed and than condensed to produce silica, alkylalkoxysilanes suchas methyltriethoxysilane silane, arylalkoxysilanes such asphenyltriethoxysilane and precursors such as triethoxysilane which yieldSiH functionality to the film. Tetrakis(methoxyethoxyethoxy)silane,tetrakis(ethoxyethoxy)silane, tetrakis(butoxyethoxyethoxy)silane,tetrakis(2-ethylethoxy)silane, tetrakis(methoxyethoxy)silane, andtetrakis(methoxypropoxy)silane are particularly useful for theinvention.

In a still further embodiment of the invention, the alkoxysilanecompounds described above may be replaced, in whole or in part, bycompounds with acetoxy and/or halogen-based leaving groups. For example,the prepolymer may be an acetoxy (CH₃—CO—O—) such as an acetoxy-silanecompound and/or a halogenated compound, e.g., a halogenated silanecompound and/or combinations thereof. For the halogenated prepolymersthe halogen is, e.g., Cl, Br, I and in certain aspects, will optionallyinclude F. Preferred acetoxy-derived prepolymers include, e.g.,tetraacetoxysilane, methyltriacetoxysilane and/or combinations thereof.

In one particular embodiment of the invention, the silicon containingprepolymer includes a monomer or polymer precursor, for example,acetoxysilane, an ethoxysilane, methoxysilane and/or combinationsthereof.

In a more particular embodiment of the invention, the silicon containingprepolymer includes a tetraacetoxysilane, a C₁ to about C₆ alkyl oraryl-triacetoxysilane and combinations thereof. In particular, asexemplified below, the triacetoxysilane is a methyltriacetoxysilane.

The silicon containing prepolymer is preferably present in the overallcomposition in an amount of from about 10 weight percent to about 80weight percent, preferably present in the overall composition in anamount of from about 20 weight percent to about 60 weight percent.

The composition then contains at least one metal-ion-free catalyst whichis an onium compound or a nucleophile. The catalyst may be, for examplean ammonium compounds, an amine, a phosphonium compound or a phosphinecompound. Non-exclusive examples of such include tetraorganoammoniumcompounds and tetraorganophosphonium compounds includingtetramethylammonium acetate, tetramethylammonium hydroxide,tetrabutylammonium acetate, triphenylamine, trioctylamine,tridodecylamine, triethanolamine, tetramethylphosphonium acetate,tetramethylphosphonium hydroxide, triphenylphosphine,trimethylphosphine, trioctylphosphine, and combinations thereof. Thecomposition may comprise a non-metallic, nucleophilic additive whichaccelerates the crosslinking of the composition. These include dimethylsulfone, dimethyl formamide, hexamethylphosphorous triamide (HMPT),amines and combinations thereof. The catalyst is preferably present inthe overall composition in an amount of from about 1 ppm by weight toabout 1000 parts per million (ppm), preferably present in the overallcomposition in an amount of from about 6 ppm to about 200 ppm.

The composition then contains at least one porogen which is a compoundor oligomer or polymer and is selected so that, does not bond to thesilicon containing pre-polymer. When it is removed, e.g., by theapplication of heat, a silica dielectric film is produced that has ananometer scale porous structure. The scale of the pores produced byporogen removal is proportional to the effective steric diameters of theselected porogen component. The need for any particular pore size range(i.e., diameter) is defined by the scale of the semiconductor device inwhich the film is employed. Furthermore, the porogen should not be sosmall as to result in the collapse of the produced pores, e.g., bycapillary action within such a small diameter structure, resulting inthe formation of a non-porous (dense) film. Further still, there shouldbe minimal variation in diameters of all pores in the pore population ofa given film. It is preferred that porogen is a compound that has asubstantially homogeneous molecular weight and molecular dimension, andnot a statistical distribution or range of molecular weights, and/ormolecular dimensions, in a given sample. The avoidance of anysignificant variance in the molecular weight distribution allows for asubstantially uniform distribution of pore diameters in the film treatedby the inventive processes. If the produced film has a wide distributionof pore sizes, the likelihood is increased of forming one or more largepores, i.e., bubbles, that could interfere with the production ofreliable semiconductor devices.

Furthermore, the porogen should have a molecular weight and structuresuch that it is readily and selectively removed from the film withoutinterfering with film formation. This is based on the nature ofsemiconductor devices, which typically have an upper limit to processingtemperatures. Broadly, a porogen should be removable from the newlyformed film at temperatures below, e.g., about 450° C. In particularembodiments, depending on the desired post film formation fabricationprocess and materials, the porogen is selected to be readily removed attemperatures ranging from about 150° C. to about 450° C. during a timeperiod ranging, e.g., from about 30 seconds to about 60 minutes. Theremoval of the porogen may be induced by heating the film at or aboveatmospheric pressure or under a vacuum, or by exposing the film toradiation, or both.

Porogens which meet the above characteristics include those compoundsand polymers which have a boiling point, sublimation temperature, and/ordecomposition temperature (at atmospheric pressure) range, for example,from about 150° C. to about 450° C. In addition, porogens suitable foruse according to the invention include those having a molecular weightranging, for example, from about 100 to about 50,000 amu, and morepreferably in the range of from about 100 to about 3,000 amu.

Porogens which do not bond to the silicon containing pre-polymer includea poly(alkylene)diether, a poly(arylene)diether, poly(cyclicglycol)diether, Crown ethers, polycaprolactone, fully end-cappedpolyalkylene oxides, fully end-capped polyarylene oxides, polynorbene,and combinations thereof. Preferred porogens which do not bond to thesilicon containing pre-polymer include poly(ethylene glycol)dimethylethers, poly(ethylene glycol)bis(carboxymethyl) ethers, poly(ethyleneglycol)dibenzoates, poly(ethylene glycol)diglycidyl ethers, apoly(propylene glycol)dibenzoates, poly(propylene glycol)diglycidylethers, poly(propylene glycol)dimethyl ether, 15-Crown 5, 18-Crown-6,dibenzo-18-Crown-6, dicyclohexyl-18-Crown-6, dibenzo-15-Crown-S andcombinations thereof.

The porogen which does not bond to the silicon containing pre-polymer ispreferably present in the overall composition, in an amount ranging fromabout 1 to about 50 weight percent, or more. More preferably the porogenis present in the composition, in an amount ranging from about 2 toabout 20 weight percent.

The composition may optionally contain an additional porogen wherein theadditional porogen has a boiling point, sublimation point ordecomposition temperature ranging from about 150° C. to about 450° C.Preferably the additional porogen has a molecular weight ranging fromabout 100 to about 50,000 amu. Preferably the additional porogencomprises a reagent comprising at least one reactive hydroxyl or aminofunctional group, and said reagent is selected from the group consistingof an organic compound, an organic polymer, an inorganic polymer andcombinations thereof. Such additional porogens suitable for use in theprocesses and compositions of the invention include polymers, preferablythose which contain one or more reactive groups, such as hydroxyl oramino.

Within these general parameters, a suitable polymer porogen for use inthe compositions and methods of the invention is, e.g., a polyalkyleneoxide, a monoether of a polyalkylene oxide, an aliphatic polyester, anacrylic polymer, an acetal polymer, a poly(caprolactone), apoly(valeractone), a poly(methyl methacrylate), a poly (vinylbutyral)and/or combinations thereof. When the porogen is a polyalkylene oxidemonoether, one particular embodiment is a C₁ to about C₆ alkyl chainbetween oxygen atoms and a C₁ to about C₆ alkyl ether moiety, andwherein the alkyl chain is substituted or unsubstituted, e.g.,polyethylene glycol monomethyl ether, or polypropylene glycol monomethylether.

Without meaning to be bound by any theory or hypothesis as to how theinvention might operate, it is believed that porogens that are, “readilyremoved from the film” undergo one or a combination of the followingevents: (1) physical evaporation of the porogen during the heating step,(2) degradation of the porogen into more volatile molecular fragments,(3) breaking of the bond(s) between the additional porogen and the Sicontaining component, and subsequent evaporation of the porogen from thefilm, or any combination of modes 1-3. The porogen is heated until asubstantial proportion of the porogen is removed, e.g., at least about50% by weight, or more, of the porogen is removed. More particularly, incertain embodiments, depending upon the selected porogen and filmmaterials, at least about 75% by weight, or more, of the porogen isremoved. Thus, by “substantially” is meant, simply by way of example,removing from about 50% to about 75%, or more, of the original porogenfrom the applied film.

An additional porogen, when employed, is preferably present in theoverall composition, in an amount ranging from about 1 to about 50weight percent. More preferably the additional porogen, when employed,is present in the composition, in an amount ranging from about 2 toabout 20 weight percent.

The overall composition then optionally includes a solvent composition.Reference herein to a “solvent” should be understood to encompass asingle solvent, polar or nonpolar and/or a combination of compatiblesolvents forming a solvent system selected to solubilize the overallcomposition components. A solvent is optionally included in thecomposition to lower its viscosity and promote uniform coating onto asubstrate by art-standard methods (e.g., spin coating, spray coating,dip coating, roller coating, and the like).

In order to facilitate solvent removal, the solvent is one which has arelatively low boiling point relative to the boiling point of anyselected porogen and the other precursor components. For example,solvents that are useful for the processes of the invention have aboiling point ranging from about 50 to about 250° C. to allow thesolvent to evaporate from the applied film and leave the active portionof the precursor composition in place. In order to meet various safetyand environmental requirements, the solvent preferably has a high flashpoint (generally greater than 40° C.) and relatively low levels oftoxicity. A suitable solvent includes, for example, hydrocarbons, aswell as solvents having the functional groups C—O—C (ethers), —CO—O(esters), —CO— (ketones), —OH (alcohols), and —CO—N-(amides), andsolvents which contain a plurality of these functional groups, andcombinations thereof.

Without limitation, solvents for the composition include di-n-butylether, anisole, acetone, 3-pentanone, 2-heptanone, ethyl acetate,n-propyl acetate, n-butyl acetate, ethyl lactate, ethanol, 2-propanol,dimethyl acetamide, propylene glycol methyl ether acetate, and/orcombinations thereof. It is preferred that the solvent does not reactwith the silicon containing prepolymer component.

The solvent component is preferably present in an amount of from about10% to about 95% by weight of the overall composition. A more preferredrange is from about 20% to about 75% and most preferably from about 20%to about 60%. The greater the percentage of solvent employed, thethinner is the resulting film. The greater the percentage of porogenemployed, the greater is the resulting porosity.

In another embodiment of the invention the composition may compriseswater, either liquid or water vapor. For example, the overallcomposition may be applied to a substrate and then exposed to an ambientatmosphere that includes water vapor at standard temperatures andstandard atmospheric pressure. Optionally, the composition is preparedprior to application to a substrate to include water in a proportionsuitable for initiating aging of the precursor composition, withoutbeing present in a proportion that results in the precursor compositionaging or gelling before it can be applied to a desired substrate. By wayof example, when water is mixed into the precursor composition it ispresent in a proportion wherein the composition comprises water in amolar ratio of water to Si atoms in the silicon containing prepolymerranging from about 0.1:1 to about 50:1. A more preferred range is fromabout 0.1:1 to about 10:1 and most preferably from about 0.5:1 to about1.5:1.

Those skilled in the art will appreciate that specific temperatureranges for crosslinking and porogen removal from the nanoporousdielectric films will depend on the selected materials, substrate anddesired nanoscale pore structure, as is readily determined by routinemanipulation of these parameters. Generally, the coated substrate issubjected to a treatment such as heating to effect crosslinking of thecomposition on the substrate to produce a gelled film.

Crosslinking may be done in step (c) by heating the film at atemperature ranging from about 100° C. to about 250° C., for a timeperiod ranging from about 30 seconds to about 10 minutes to gel thefilm. The artisan will also appreciate that any number of additionalart-known curing methods are optionally employed, including theapplication of sufficient energy to cure the film by exposure of thefilm to electron beam energy, ultraviolet energy, microwave energy, andthe like, according to art-known methods.

Once the film has aged, i.e., once it is is sufficiently condensed to besolid or substantially solid, the porogen can be removed. The lattershould be sufficiently non-volatile so that it does not evaporate fromthe film before the film solidifies. The porogen is removed in a step(d) by heating the gelled film at a temperature ranging from about 150°C. to about 450° C., preferably from about 150° C. to about 350° C. fora time period ranging from about 30 seconds to about 1 hour. Animportant feature of the invention is that the step (c) crosslinking isconducted at a temperature which is less than the heating temperature ofstep (d).

Utility

The present composition may also comprise additional components such asadhesion promoters, antifoam agents, detergents, flame retardants,pigments, plasticizers, stabilizers, and surfactants. The presentcomposition has utility in non-microelectronic applications such asthermal insulation, encapsulant, matrix materials for polymer andceramic composites, light weight composites, acoustic insulation,anti-corrosive coating, binders for ceramic powders, and fire retardantcoating.

The present composition is particularly useful in microelectronicapplications as a dielectric substrate material in microchips, multichipmodules, laminated circuit boards, or printed wiring boards. The presentcomposition may also be used as an etch stop or hardmask.

The present films may be formed by solution techniques such as spraying,rolling, dipping, spin coating, flow coating, or casting, with spincoating being preferred for microelectronics. Preferably, the presentcomposition is dissolved in a solvent. Suitable solvents for use in suchsolutions of the present compositions include any suitable pure ormixture of organic, organometallic, or inorganic molecules that arevolatized at a desired temperature. Suitable solvents include aproticsolvents, for example, cyclic ketones such as cyclopentanone,cyclohexanone, cycloheptanone, and cyclooctanone; cyclic amides such asN-alkylpyrrolidinone wherein the alkyl has from about 1 to 4 carbonatoms; and N-cyclohexylpyrrolidinone and mixtures thereof. A widevariety of other organic solvents may be used herein insofar as they areable to aid dissolution of the adhesion promoter and at the same timeeffectively control the viscosity of the resulting solution as a coatingsolution. Various facilitating measures such as stirring and/or heatingmay be used to aid in the dissolution. Other suitable solvents includemethyethylketone, methylisobutylketone, dibutyl ether, cyclicdimethylpolysiloxanes, butyrolactone, γ-butyrolactone, 2-heptanone,ethyl 3-ethoxypropionate, 1-methyl-2-pyrrolidinone, propylene glycolmethyl ether acetate (PGMEA), and hydrocarbon solvents such asmesitylene, xylenes, benzene, and toluene.

The present composition may be used in electrical devices and morespecifically, as an interlayer dielectric in an interconnect associatedwith a single integrated circuit (“IC”) chip. An integrated circuit chiptypically has on its surface a plurality of layers of the presentcomposition and multiple layers of metal conductors. It may also includeregions of the present composition between discrete metal conductors orregions of conductor in the same layer or level of an integratedcircuit.

The present films may be used in dual damascene (such as copper)processing and substractive metal (such as aluminum oraluminum/tungsten) processing for integrated circuit manufacturing. Thepresent composition may be used in a desirable all spin-on stacked filmas taught by Michael E. Thomas, “Spin-On Stacked Films for Low k_(eff)Dielectrics”, Solid Slate Technology (July 2001), incorporated herein inits entirety by reference. The present composition may be used in an allspin-on stacked film having additional dielectrics such as taught bycommonly assigned U.S. Pat. Nos. 6,248,457B1; 5,986,045; 6,124,411; and6,303,733.

Analytical Test Methods

Dielectric Constant: The dielectric constant was determined by coating athin film of aluminum on the cured layer and then doing acapacitance-voltage measurement at 1 MHz and calculating the k valuebased on the layer thickness.

Refractive Index: The refractive index measurements were performedtogether with the thickness measurements using a J. A. Woollam M-88spectroscopic ellipsometer. A Cauchy model was used to calculate thebest fit for Psi and Delta. Unless noted otherwise, the refractive indexwas reported at a wavelenth of 633 nm (details on Ellipsometry can befound in e.g. “Spectroscopic Ellipsometry and Reflectometry” by H. G.Thompkins and William A. McGahan, John Wiley and Sons, Inc., 1999).

Average Pore Size Diameter: The N₂ isotherms of porous samples wasmeasured on a Micromeretics ASAP 2000 automatic isothermal N₂ sorptioninstrument using UHP (ultra high purity industrial gas) N₂, with thesample immersed in a sample tube in a liquid N₂ bath at 77° K.

For sample preparation, the material was first deposited on siliconwafers using standard processing conditions. For each sample, threewafers were prepared with a film thickness of approximately 6000Angstroms. The films were then removed from the wafers by scraping witha razor blade to generate powder samples. These powder samples werepre-dried at 180° C. in an oven before weighing them, carefully pouringthe powder into a 10 mm inner diameter sample tube, then degassing at180° C. at 0.01 Torr for >3 hours.

The adsorption and desorption N₂ sorption was then measuredautomatically using a 5 second equilibration interval, unless analysisshowed that a longer time was required. The time required to measure theisotherm was proportional to the mass of the sample, the pore volume ofthe sample, the number of data points measured, the equilibrationinterval, and the P/Po tolerance. (P is the actual pressure of thesample in the sample tube. Po is the ambient pressure outside theinstrument.) The instrument measures the N₂ isotherm and plots N₂ versusP/Po.

The apparent BET (Brunauer, Emmett, Teller method for multi-layer gasabsorption on a solid surface disclosed in S. Brunauer, P. H. Emmett, E.Teller; J. Am. Chem. Soc. 60, 309-319 (1938)) surface area wascalculated from the lower P/Po region of the N2 adsorption isothermusing the BET theory, using the linear section of the BET equation thatgives an R² fit >0.9999.

The pore volume was calculated from the volume of N₂ adsorbed at therelative pressure P/Po value, usually P/Po˜0.95, which is in the flatregion of the isotherm where condensation is complete, assuming that thedensity of the adsorbed N₂ is the same as liquid N₂ and that all thepores are filled with condensed N₂ at this P/Po.

The pore size distribution was calculated from the adsorption arm of theN₂ isotherm using the BJH (E. P. Barret, L. G. Joyner, P. P. Halenda; J.Am. Chem. Soc., 73, 373-380 (1951)) theory. This uses the Kelvinequation, which relates curvature to suppression of vapor pressure, andthe Halsey equation, which describes the thickness of the adsorbed N₂monolayer versus P/Po, to convert the volume of condensed N₂ versus P/Poto the pore volume in a particular range of pore sizes.

The average cylindrical pore diameter D was the diameter of a cylinderthat has the same apparent BET surface area Sa (m²/g) and pore volume Vp(cc/g) as the sample, so D (nm)=4000 Vp/Sa.

FTIR: FTIR spectra were taken using a Nicolet Magna 550 FTIRspectrometer in transmission mode. Substrate background spectra weretaken on uncoated substrates. Film spectra were taken using thesubstrate as background. Film spectra were then analyzed for change inpeak location and intensity.

The following non-limiting examples serve to illustrate the invention.

EXAMPLE 1

This example shows the production of a nanoporous silica with a porogenhaving reactive end groups. A precursor was prepared by combining, in a100 ml round bottom flask (containing a magnetic stirring bar), 10 gtetraacetoxysilane, 10 g methyltriacetoxysilane, and 17 g propyleneglycol methyl ethyl acetate (PGMEA). These ingredients were combinedwithin an N₂-environment (N₂ glove bag). The flask was also connected toan N₂ environment to prevent environmental moisture from entering thesolution (standard temperature and pressure).

The reaction mixture was heated to 80° C. before 1.5 g of water wasadded to the flask. After the water addition is complete, the reactionmixture was allowed to cool to ambient before 4.26 g of polyethyleneglycol monomethylether (“PEO”; MW550 amu) was added as a porogen andstirring continued for another 2 hrs. Thereafter, the resulting solutionwas filtered through a 0.2 micron filter to provide the precursorsolution masterbatch for the next step. The solution was then depositedonto a series of 8-inch silicon wafers, each on a spin chuck and spun at1000 rpm for 30 seconds. The presence of water in the precursor resultedin the film coating being substantially condensed by the time that thewafer was inserted into the first oven. Insertion into the first oven,as discussed below, took place within the 10 seconds of the completionof spinning. Each coated wafer was then transferred into a sequentialseries of ovens preset at specific temperatures, for one minute each. Inthis example, there are three ovens, and the preset oven temperatureswere 125° C., 200° C., and 350° C., respectively. The PEO was driven offby these sequential heating steps as each wafer was moved through eachof the three respective ovens. Each wafer was cooled after receiving thethree-oven stepped heat treatment, and the produced dielectric film wasmeasured using ellipsometry to determine its thickness and refractiveindex. Each film-coated wafer was then further cured at 425° C. for onehour under flowing nitrogen.

The film has a cure thickness of 6770 Å and a cure refractive index of1.230. The cured film produced has a Δk % of about 10% (see entry 1 ofthe following table). In the table, capacitance of the film was measuredunder ambient conditions (room temperature and humidity). Dielectricconstant based on ambient capacitance value is called k ambient. Thecapacitance of the film was measured again after heating the wafer in ahot plate at 200° C. for 2 minutes in order to drive off adsorbedmoisture. Dielectric constant based on the de-moisture capacitance iscalled k de-gas. The differences of the two k values, calculated from Δk%=(k_(ambient)−k_(degas))/k_(ambient)*100%, is one of the indication onthe hydrophobicity of a given film. Various amounts (5.43 and 5.04 g forentry 2 and 3, respectively) of PEO were added to produce nanoporousfilms of different dielectric constant, and the Δk % is greater than10%. (See entries 2 and 3 of the following table) Cured films were alsomeasured using FTIR to determine silicon methyl (SiC, ν: 1277.4cm⁻¹)-to-silicon oxide (SiO, ν: 1055.0 cm⁻¹) area ratio. In general, theSiC/SiO ratio appears to be in the range of 0.0250 and 0.0264. Theaverage pore size diameters was 2.1-2.2 nm.

EXAMPLE 2

Example 1 is repeated except this example uses poly(ethyleneglycol)dimethyl ether (“DMEPEO”; MW500 amu) as the porogen. This exampleshows that a more hydrophobic film is obtained. In one case, smallamount of methyltriacetoxysilane (MTAS, 2%, see entry 6) was added tothe solution to serve as an in-situ surface modifier to make the surfaceless hydrophilic. Films were deposited onto a wafer by spin coating at1000 rpm or 1500 rpm. After spin coating, the film was heated in threehot plates at temperatures of 125° C., 200° C. and 350° C., one minuteeach. After bake, the film was cured under flowing nitrogen at 425° C.for one hour. The results of k, R.I. and SiC/SiO of the post-cure filmsare listed in the following table. It is shown that the Δk % value isbelow 8.5%, and SiC/SiO ratio is 0.0282±0.0001. (See entries 4-65) Whenextra MTAS (2%) was added, the resulting film was significantly morehydrophobic having a Δk % value of 2.2%. (See entry 6).

Cured SiC/SiO Porogen Type Refractive Dielectric Constant FTIR EntryType Amount (g)* Index k_(ambient) k_(de-gas) Δ k % Ratio 1 PEO 4.261.230 2.55 2.29 10.20 0.0264 2 PEO 5.43 1.210 2.59 2.25 13.13 0.0258 3PEO 5.04 1.225 2.49 2.18 12.45 0.0250 4 DMEPEO 5.43 1.203 2.21 2.02 8.600.0280 5 DMEPEO 6.98 1.171 1.98 1.87 5.56 0.0283 6 DMEPEO* 4.26 1.2052.23 2.18 2.24 0.0289

The amount of porogen is based on a batch size of 10 gtetraacetoxysilane and 10 g methyltriacetoxysilane.

Pore size/volume data Average Pore Size Entry Diameter (nm) Pore Volume(cc/g) 1 2.10 0.564 2 2.17 0.527 3 2.20 0.744 4 2.85 0.837 5 3.48 0.9776 N/A N/A

While the present invention has been particularly shown and describedwith reference to preferred embodiments, it will be readily appreciatedby those of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. It is intended that the claims be interpreted to coverthe disclosed embodiment, those alternatives which have been discussedabove and all equivalents thereto.

1. A method of producing a nanoporous silica dielectric film comprising(a) preparing a composition comprising a silicon containing pre-polymer,a metal-ion-free catalyst selected from the group consisting oftetramethylammonium acetate, tetrabutylammonium acetate,tetramethylphosphonium acetate, tetramethylphosphonium hydroxide,triphenylphosphine, trimethylphosphine, trioctylphosphine, andcombinations thereof; and a porogen which does not bond to the siliconcontaining pre-polymer; (b) coating a substrate with the composition toform a film, (c) crosslinking thc composition to produce a gelled film,and (d) heating the gelled film at a temperature and for a durationeffective to remove substantially all of said porogen.
 2. The method ofclaim 1 wherein the nanoporous silica dielectric film has a pore voidvolume of from about 5% to about 80% based on the volume or the film. 3.The method of claim 1 wherein the resulting nanoporous silica dielectricfilm has a dielectric constant of about 3 or below.
 4. The method ofclaim 1 wherein the nanoporous silica dielectric film has an averagepore diameter in the range of from about 1 nm to about 30 nm.
 5. Themethod of claim 1 wherein the porogen is selected from the groupconsisting of a poly(alkylene) diether, a poly(arylene) diether,poly(cyclic glycol) diether, Crown ethers, polycaprolactone, fullyend-capped polyalkylene oxides, fully end-capped polyarylene oxides,polynorbene, and combinations thereof.
 6. The method of claim 1 whereinthe porogen is selected from the group consisting of a poly(ethyleneglycol) dimethyl ether, a poly(ethylene glycol) bis(carboxymethyl)ether, a poly(ethylene glycol) dibenzoate, a poly(ethylene glycol)propylmethyl ether, a poly(ethylene glycol) diglycidyl ether, apoly(propylene glycol) dibenzoate, a poly(propylene glycol) dibutylether, a poly(propylene glycol) dimethyl ether, a poly(propylene glycol)diglycidyl ether, 15-Crown 5, 18-Crown-6, dibenzo-18-Crown-6,dicyclohexyl-18-Crown-6, dibenzo-15-Crown-5 and combinations thereof. 7.The method of claim 1 wherein the composition further comprises anon-metallic, nucleophilic additive which accelerates the crosslinkingof the composition.
 8. The method of claim 1 wherein the compositionfurther comprises a nucleophilic additive which accelerates thecrosslinking of the composition, which is selected from the groupconsisting of dimethyl sulfone, dimethyl formamide,hexamethylphosphorous triamide, amines and combinations thereof.
 9. Themethod of claim 1 wherein the composition further comprises water in amolar ratio of water to Si atoms in said silicon containing pre-polymerranging from about 0.1:1 to about 50:1.
 10. The method of claim 1wherein the composition comprises a silicon containing pre-polymer ofFormula I:Rx-Si-Ly  (Formula I) wherein x is an integer ranging from 0 to about 2,and y is 4-x, an integer ranging from about 2 to about 4; R isindependently selected from the group consisting of alkyl, aryl,hydrogen, alkylene, arylene, and combinations thereof; L is anelectronegative moiety, independently selected from the group consistingof alkoxy, carboxy, amino, amido, halide, isocyanato and combinationsthereof.
 11. The method of claim 10 wherein the composition comprises apolymer formed by condensing a pre-polymer according to Formula I,wherein the number average molecular weight of said polymer ranges fromabout 150 to about 300,000 amu.
 12. The method of claim 1 wherein thecomposition comprises a silicon containing pre-polymer selected from thegroup consisting of an acetoxysilane, an ethoxysilane, a methoxysilane,and combinations thereof.
 13. The method of claim 1 wherein thecomposition comprises a silicon containing pre-polymer selected from thegroup consisting of tetraacetoxysilane, a C₁ to about C₆ alkyl oraryl-triacetoxysilane, and combinations thereof.
 14. The method of claim13 wherein said triacetoxysilane is methyltriacetoxysilane.
 15. Themethod of claim 1 wherein the composition comprises a silicon containingpre-polymer selected from the group consisting oftetrakis(2,2,2-trifluoroethoxy)silane, tetrakis(trifluoroacetoxy)silane,tetraisocyanatosilane, tris(2,2,2-trifluoroethoxy)methylsilane,tris(trifluoroacetoxy)methylsilane, methyltriisocyanatosilane andcombinations thereof.
 16. The method of claim 1 wherein step (c)comprises a crosslinking which is conducted at a temperature which isless than the heating temperature of step (d).
 17. The method of claim 1further comprising an additional porogen wherein the additional porogenhas a boiling point, sublimation point or decomposition temperatureranging from about 150° C. to about 450° C.
 18. The method of claim 1wherein step (c) comprises heating the film at a temperature rangingfrom about 100° C. to about 250° C., for a time period ranging fromabout 30 seconds to about 10 minutes.
 19. The method of claim 1 whereinstep (d) comprises heating the film at temperature ranging from about150° C. to about 450° C., for a time period ranging from about 30seconds to about 1 hour.
 20. The method of claim 1 further comprising anadditional porogen wherein the additional porogen has a molecular weightranging from about 100 to about 50,000 amu.
 21. The method of claim 1further comprising an additional porogen wherein the additional porogencomprises a reagent comprising at least one reactive hydroxyl or aminofunctional group, and said reagent is selected from the group consistingof an organic compound, an organic polymer, an inorganic polymer andcombinations thereof.
 22. The method of claim 1 further comprising anadditional porogen wherein the additional porogen is selected from thegroup consisting of a polyalkylene oxide, a monoether of a polyalkyleneoxide, an aliphatic polyester, an acrylic polymer, an acetal polymer, apoly(caprolatactone), a poly(valeractone), a poly(methyl methacrylate),a poly (vinylbutyral) and combinations thereof.
 23. The method of claim1 further comprising an additional porogen wherein the additionalporogen comprises a polyalkylene oxide monoether which comprises a C₁ toabout C₆ alkyl chain between oxygen atoms and a C₁ to about C₆ alkylether moiety, and wherein the alkyl chain is substituted orunsubstituted.
 24. The method of claim 23 wherein the polyalkylene oxidemonoether is a polyethylene glycol monomethyl ether or polypropyleneglycol monobutyl ether.
 25. The method of claim 1 wherein the porogen ispresent in the composition in an amount of from about 1 to about 50percent by weight of the composition.
 26. The method of claim 1 whereinthe composition further comprises a solvent.
 27. The method of claim 1wherein the composition further comprises solvent in an amount rangingfrom about 10 to about 95 percent by weight of the composition.
 28. Themethod of claim 1 wherein the composition further comprises a solventhaving a boiling point ranging from about 50 to about 250° C.
 29. Themethod of claim 1 wherein the composition further comprises a solventselected from the group consisting of hydrocarbons, esters, ethers,ketones, alcohols, amides and combinations thereof.
 30. The method ofclaim 26 wherein the solvent is selected from the group consisting ofdi-n-butyl ether, anisole, acetone, 3-pentanone, 2-heptanone, ethylacetate, n-propyl acetate, n-butyl acetate, 2-propanol, dimethylacetamide, propylene glycol methyl ether acetate, and combinationsthereof.
 31. A nanoporous silica dielectric film formed from acomposition comprising a silicon containing pre-polymer, a metal ionfree catalyst selected from the group consisting of tetramethylammoniumacetate, tetrabutylammonium acetate, tetramethylphosphonium acetate,tetramethylphosphonium hydroxide, triphenylphosphine,trimethylphosphine, trioctylphosphine, and combinations thereof; and aporogen that does not bond to the silicon containing pre-polymer and isselected from the group consisting of poly(alkylene) diether, apoly(arylene) dieter, poly(cyclic glycol) diether, Crown ethers,polycaprolactone, fully end-capped polyalkylene oxides, fully end-cappedpolyarylene oxides, polynorbene, and combinations thereof, whichnanoporous silica dielectric film is produced according to the method ofclaim
 1. 32. A semiconductor device comprising a nanoporous dielectricfilm of claim
 31. 33. The semiconductor device of claim 32 that is anintegrated circuit.
 34. The nanoporous silica dielectric film of claim31 wherein said metal-ion-free catalyst is tetramethylammonium acetate.35. The nanoporous silica dielectric film of claim 31 wherein saidsilicon containing pre-polymer comprises a combination of acetoxy-basedleaving groups.
 36. The nanoporous silica dielectric film of claim 35wherein said silicon containing pre-polymer comprising a combination ofacetoxy-based leaving groups comprises tetraacetoxysilane andmethyltriacetoxysilane.
 37. A nanoporous silica dielectric film of claim31 wherein said composition is a spin-on composition.
 38. In a method ofcontrolling the pore size of a porous silica film, comprising (a)preparing a composition comprising a silicon containing pre-polymer, ametal-ion-free catalyst selected from the group consisting of;tetramethylammonium acetate, tetrabutylammonium acetate,tetramethylphosphonium acetate, tetramethylphosphonium hydroxide,triphenylphosphine, trimethylphosphine, trioctylphosphine, andcombinations thereof; and a porogen; (b) coating a substrate with thecomposition to form a film, (c) crosslinking the composition to producea gelled film, and (d) heating the gelled film at a temperature and fora duration effective to remove substantially all of said porogen; themethod comprising using a porogen which does not bond to the siliconcontaining pre-polymer.